In a system typically represented by a wireless sensor network in which a large number of wireless communication devices are installed, there is a demand for non-use of power feeding wiring in order to reduce the installation cost and to enhance the flexibility of the installation place. As a method of realizing this demand, a method using wireless power transmission (wireless power feeding or rectenna) technology is available which feeds power using radio waves. The wireless power transmission technology is sometimes called wireless power feeding or rectenna.
There is a method that uses battery driving of a wireless communication device or power generation utilizing energy existing in the surrounding environment as the power source of the wireless communication device.
However, with the method that uses such battery driving or power generation as the power source, the wireless communication device must be able to operate continuously for a long time with a limited amount of electric power. Therefore, it is required for the wireless communication device to minimize its power consumption needed during operation.
Patent Literature 1 discloses a non-contact power transmission system. In the power transmission system of Patent Literature 1, the transmission time of electric power from a power transmission unit to a power receiving apparatus is segmented into a plurality of periods at predetermined intervals. Then, each power receiving apparatus is allocated to one of the segmented periods, and electric power is transmitted in the allocated period. Patent Literature 1 states this method enables stable electric power transmission to each power receiving apparatus.
However, when the power receiving apparatus is a sensor terminal, it is necessary to minimize the battery consumption of the sensor terminal. In the case of using wireless power transmission, it is necessary to minimize the power consumption in addition to stably feed electric power to the power receiving apparatus.
When slave stations operate intermittently, if the slave stations adopt time management of RTC (Real Time Clock), the intermittent cycles of the slave stations may undesirably be shifted from each other. Therefore, when the slave stations communicate in a time-sync manner, the communication timings are shifted. Then, the number of times of synchronization re-locking increases, leading to an increased power consumption.